Nanocomposites are polymers reinforced with nanometer sized particles, i.e., particles with a dimension on the order of 1 to several hundred nanometers. Polymer-layered silicate nanocomposites normally incorporate a layered clay mineral filler in a polymer matrix. Layered silicates are made up of several hundred thin platelet layers stacked into an orderly packet known as a tactoid. Each of these platelets is characterized by large aspect ratio (diameter/thickness on the order of 100-1000). Accordingly, when the clay is dispersed homogeneously and exfoliated as individual platelets throughout the polymer matrix, dramatic increases in strength, flexural and Young's modulus, and heat distortion temperature are observed at very low filler loadings (<10% by weight) because of the large surface area contact between polymer and filler. These materials can be used, for example, in structural, semi-structural, high heat underhood, and Class A automotive components, among others
To improve the interaction between hydrophilic fillers and a hydrophobic polymer matrix, the surface of the particle may be pretreated with an agent intended to make the surface more organophilic, for example, a quaternary alkylammonium salt for clays and organosilanes for glass fibers or other glass fillers
U.S. Patent Publication 2006/0205856 teaches an in situ polymerization approach for the preparation of polyester nanocomposite compositions, namely, a process for manufacturing a thermoplastic polyester nanocomposite comprising mixing a sepiolite-type clay with at least one thermoplastic polyester precursor selected from the group consisting of at least one diacid or diester and at least one diol; at least one polymerizable polyester monomer; at least one linear polyester oligomer; and at least one macrocyclic polyester oligomer; and subsequently polymerizing said at least one polyester precursor in the presence or absence of a solvent.
Polyester nanocomposites produced by means of in situ polymerization comprise polyester having a high concentration of carboxylic acid end groups. In general, high acid end group content is undesirable in polyesters because of deleterious effects on properties such as hydrolytic and melt stability. High acid end group content also leads to corrosion problems when the polyester is in contact with metal and an outgassing problem at elevated temperature, which is particularly important in certain automotive applications. Furthermore, the acid end groups can catalyze the hydrolysis of the ester bonds of polyesters in an auto-catalytic process, leading to lower molecular weight and embrittlement. In general, the acid end content of polyesters is controlled by choosing proper catalysts, adjusting the amount of the catalysts, using specially designed reactors, and changing process conditions.
Among polyesters, high acid end group content is particularly deleterious in poly(butylene terephthalate) (“PBT”). For many applications of PBT in particular, such as automobile lighting, low acid end content, and good elongation at break. Therefore, it is highly desired to have a simple method to lower the acid end content of PBT-filler composites using the same processing equipment and conditions as thermoplastic polyesters without significantly decreasing elongation at break
For the reasons set forth above, there exists a need for a simple process for producing filled polyesters, especially polyester nanocomposites, having low acid end group content.